Cutting tools in the form of, e.g., indexable inserts having cutting edges made of polycrystalline cubic boron nitride (PcBN) have been used for many years in chip forming machining of ferrous materials. Attractive properties of the PcBN include high toughness, good high temperature hardness and relatively high chemical inertness.
Normally, the PcBN used for cutting tools is a compound which consists of cubic boron nitride (cBN), mixed together with other hard constituents and/or metallic binder. The compound is sintered in a high temperature-high pressure process, often together with a cemented carbide support. Two main types of PCBN materials are presently being used for tools:
One type of PcBN material contains more than 80% cBN by weight and the remaining portion consists of a metallic binder phase, usually Co that originates in the cemented carbide part of the tool. An example of such a material is disclosed in U.S. Pat. No. 5,326,380. Tools made from this kind of, so-called, high cBN content PcBN material are often used for machining of cast irons.
Another type of PcBN material contains less cBN, about 60% cBN by weight, and the remainder is other hard and wear-resistant constituents such as carbides, nitrides, carbonitrides, oxides, borides of the metals of groups IVa to VIa of the Periodic Table, commonly Ti. An example of such a material is described in U.S. patent application Ser. No. 08/440,773. This kind of, so-called, low cBN content PcBN material is mainly used in cutting tools designed for machining of hardened steels and other hard materials.
Traditionally, PcBN tools consist of a PcBN inlay brazed into a pocket in a cemented carbide body, thus forming a cutting tool, which is subsequently ground to proper standard finished dimensions. Solders that are commonly used for brazing have melting points in the range of 600-840.degree. C., which temperature defines an upper limit of thermal load in machining operations. If the temperature at the cBN/cemented carbide body bond is increased close to or beyond the melting point of the solder, the inlay might slide and cause malfunction of the tool.
An improvement of the PcBN concept is disclosed in U.S. patent application Ser. No. 08/446,490 in which is described a metal cutting insert comprising a cemented carbide body and at least one body of superhard abrasive material, such as polycrystalline diamond (PCD) or PcBN, bonded to an edge surface of the body and extending from one side surface to the other side surface thereof. There can be a plurality of superhard bodies disposed at respective corners of the body. The abrasive material is applied to the body in a container and then sintered and simultaneously bonded to the body in an elevated pressure/temperature step. Inserts can, for instance, be made either in rod form, i.e., in one piece, which thereafter can be transversely sliced into thin inserts, or the inserts can be made in separate pieces, with or without separators within the container.
In machining with PcBN tools, problems with crater wear of the PcBN phase often occur. The cause is a combination of abrasive wear and chemical dissolution of the tool material into the chip which gradually weakens the cutting edge, a process eventually leading to edge breakage. The chemical wear is caused by a chemical reaction between the tool material and the work piece at the elevated temperatures induced in the cutting zone. In a typical machining operation using the PcBN, tool wear on the flank is usually small or very small. This renders used edges difficult to distinguish from unused ones. The small wear marks on the insert flank are difficult to observe with bare eyes.